gatt: Fix btd_gatt_read_attribute

In case there is no attrib associated to the given device, we should
return ECOMM error and generate a "Not Connected" error in D-Bus
interface.
gatt: Fix btd_gatt_write_attribute

In case there is no attrib associated to the given device, we should
return ECOMM error and generate a "Not Connected" error in D-Bus
interface.